Notch signaling regulates multiple aspects of hematopoietic development. Notch receptors are a conserved family that influence cell fates, survival, proliferation, and border formation in multi-cellular organisms. This competing renewal represents our long-term plan to understand the function of Notch signaling in hematopoietic development and function. We and others have provided evidence that Notch regulates T cell commitment from a multi-potential progenitor and regulates multiple aspects of intrathymic T cell development, The proposed studies focus on the function of Notch signaling in T cell commitment and early thymocyte development, In Specific Aim 1, we will identify the bone marrow, intra-thymic and extra-thymic progenitors upon which Notch acts to specify T cell commitment. In Specific Aim 2, we will determine the function of Notch signaling and identify the mechanism by which Notch influences multiple events during early thymocyte development. In Aim 3, we will determine the mechanism that downregulates Notch signaling at the DN3-DN4 transition. Our studies will utilize a combination of murine models, including a novel loss-of-function model created by us, organ and stromal cell cultures, and biochemical approaches. In particular, we seek to understand the function of Notch signaling at the pre-TCR checkpoint, the downstream signals by which Notch regulates intrathymic T cell development, and the mechanisms regulating Notch1 transcription. Together, these studies will lead to an improved understanding of lymphocyte development and function, and in doing so, will provide insights into modulating the immune system for therapeutic advances. [unreadable] [unreadable]